ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 157 



16. That instrnments constructed on the principle of Sir W. Thom- 

 son's Quadrant Electrometer used idiostaticallj, and for high 

 pressures instruments on the principle of the balance, electro- 

 static forces being balanced against a known weight, should 

 be adopted as Board of Trade standards for the measurement 

 of pressure, whether unvarying or alternating. 



7. We have adopted the system of electrical units originally defined 

 by the British Association for the Advancement of Science ; and we have 

 found in its recent researches, as well as in the deliberations of the Inter- 

 national Congress on Electrical Units, held in Paris, valuable guidance 

 for determining the exact magnitude of the several units of electrical 

 measurement, as well as for the verification of the material standards. 



8. We have stated the relation between the proposed standard ohm. 

 and the unit of resistance originally determined by the British Associa- 

 tion, and have also stated its relation to the mercurial standard adopted 

 by the International Conference. 



9. We find that considerations of practical importance make it un- 

 desirable to adopt a mercurial standard, we have, therefore, preferred to 

 adopt a material standard constructed in solid metal. 



10. It appears to us to be necessary that in transactions between 

 buyer and seller a legal character should henceforth be assigned to the 

 units of electrical measurement now suggested, and with this view, that 

 the issue of an Order in Council should be recommended, under the 

 Weights and Measures Act, in the form annexed to this report. 



Specification kefeeked to in Resolution 10. 



In the following specification the term silver voltameter means the 

 arrangement of apparatus by means of which an electric current is passed 

 through a solution of nitrate of silver in water. The silver voltameter- 

 measures the total electrical quantity which has passed during the time 

 of the experiment, and by noting this time the time-average of the 

 current, or, if the current has remained constant, the current itself can 

 be deduced. 



In employing the silver voltameter to measure currents of about 

 1 ampere the following arrangements should be adopted. The kathode 

 on which the silver is to be deposited should take the form of a platinum 

 bowl not less than 10 cm. in diameter, and from 4 to 6 cm. in depth. 



The anode should be a plate of pure silver some 30 square cm. in area 

 and 2 or 3 millimetres in thickness. 



This is supported horizontally in the liquid near the top of the solu- 

 tion by a platinum wire passed through holes in the plate at opposite 

 corners. To prevent the disintegrated silver which is formed on the 

 anode from falling on to the kathode, the anode should be wrapped round 

 with pure filter paper, secured at the back with sealing wax. 



The liquid should consist of a neutral solution of pure silver nitrate, 

 containing about 15 parts by weight of salt to 85 parts of water. 



The resistance of the voltameter changes somewhat as the current 

 passes. To prevent these changes having too great an effect on the- 

 current, some resistance besides that of the voltameter should be inserted 

 in the circuit. The total metallic resistance of the circuit should not be 

 less than 10 ohms. 



